Gas-shield-electrode and composite bifunctional air-electrode using the same for use in metal-air batteries

1. A method of operating a secondary Metal-Air electrochemical cell with a bulk of liquid electrolyte, a metal anode and an air cathode, the air cathode comprising cavities serving as hollow spaces on a side to be facing the anode, the method comprising the steps of:

a) at start of a charging session, creating in less than 2 seconds an oxygen gas-shield on an electrolyte side of the air cathode to be facing the anode obstructing ion passage between the bulk of the electrolyte and the air cathode, wherein oxygen forming the oxygen gas-shield evolves from the air cathode at a time of charging,

b) charging the cell without anodic polarization of the air cathode with the help of

(i) electric conductive material placed between the electrolyte side of air cathode and the bulk of electrolyte, and,

(ii) the said oxygen gas-shield obstructing passage of ions of the electrolyte between

the electrolyte side of air cathode and the bulk of electrolyte,

c) removing the oxygen gas-shield at start of a discharging session, wherein the step of removing oxygen gas-shield at start of a discharging session is performed by the air cathode consuming oxygen from its electrolyte-side under cathodic polarization.

2. An air cathode, for use in metal-air electrochemical cell with a bulk of liquid electrolyte, the air cathode comprising cavities serving as hollow spaces, on which under anodic polarization oxygen gas evolves from the electrolyte,

wherein a geometry of said air cathode is such that

said air cathode permits to perform by forming a gas-shield under anodic polarization, the following steps:

(a) at start of a charging session,

creating in less than 2 seconds an oxygen gas-shield

on an electrolyte side of the air cathode obstructing ion passage between the bulk of the electrolyte and the air cathode, wherein oxygen forming the oxygen gas-shield evolves from the air cathode at a time of charging,

(b) charging the cell without anodic polarization of the air cathode with the help of

(i) electric conductive material placed between the electrolyte side of air cathode and the bulk of electrolyte, and,

(ii) the said oxygen gas-shield obstructing passage of ions of the electrolyte between the electrolyte side of air cathode and the bulk of electrolyte, and

(c) removing the oxygen gas-shield at start of a discharging session, wherein the step of removing oxygen gas-shield at start of a discharging session is performed by the air-cathode consuming oxygen from its electrolyte side under cathodic polarization; and

while, without presence of said gas-shield, the air cathode offers sufficient passage to allow ions of the electrolyte to flow therethrough.

3. The air cathode according to claim 2 wherein material from which the air cathode is made forms a matrix with hollow spaces retaining the gas-shield.

4. The air cathode according to claim 2 herein the hollow spaces have an average surface area of between 0.04 mm 2 to 25 mm 2 .

5. The air cathode according to claim 2 wherein said air cathode comprises an alveolar matrix with walls separating an alveoli.

6. The air cathode according to claim 5 wherein the walls have an average thickness of 0.05 mm to 0.5 mm.

7. The air cathode according to claim 3 , further comprising a grid forming channels to guide oxygen bubbles that flow during anodic polarization.

8. The air cathode according to claim 7 wherein the grid forming channels comprise sides, and at least a portion of the sides of said grid forming channels is conductive.

9. The air cathode according to claim 2 wherein said air cathode has a thickness of 0.2 mm to 3 mm.

10. The method according to claim 1 , wherein the air cathode comprises cavities that are formed from an alveolar matrix with walls separating an alveoli.

11. The method according to claim 10 , wherein the walls have an average thickness of 0.05 mm to 0.5 mm.

12. The method according to claim 10 , wherein the air-cathode further comprises a grid forming channels to guide oxygen bubbles that flow during anodic polarization.

13. The method according to claim 12 , wherein the grid forming channels comprises sides, and at least a portion of the sides is conductive.

14. The method according to claim 10 wherein the air-cathode has a thickness of 0.2 mm to 3 mm.